DOI: 10.1002/chem.201203771 A Versatile Methodology for the Controlled Synthesis of Photoluminescent High-Boron-Content Dendrimers ArƁntzazu GonzƁlez-Campo, [a] Albert Ferrer-Ugalde, [a] Clara ViÇas, [a] Francesc Teixidor, [a] Reijo SillanpȨȨ, [b] Jesffls Rodrȷguez-Romero, [c] Rosa Santillan, [c] Norberto FarfƁn, [d] and Rosario NfflÇez* [a] Introduction The search for luminescent materials is continuously ex- panding due to their applications as light-emitting diodes, semiconductor lasers, probes, or fluorescence sensors, among others. [1] In boron chemistry, there are few reports concerning the photophysical properties of boranes and car- boranes. Nevertheless, during the last few years, the interest in the development of new photoluminescent materials that incorporate polyhedral boron clusters has increased, as shown by the incorporation of these compounds, which pos- sess special electronic properties and geometry, into fluores- cent systems. [2] For this reason, different fluorescent carACHTUNGTRENNUNGbo- ACHTUNGTRENNUNGrane-appended star-shaped molecules, in which the carACHTUNGTRENNUNGbo- ACHTUNGTRENNUNGrane is linked to a fluorescent p-conjugated core, [3] to p-con- jugated organic systems, [4] or polymers, [5, 6] have been recent- ly described, and the influence of the cluster on the emission properties studied. On the other hand, the interest in the use of different types of platforms for the incorporation of boron-based clus- ters to obtain high-boron-content molecules for boron neu- tron capture therapy (BNCT), or for drug delivery, has in- creased enormously. [3, 7] During the last decade, our group has contributed to the development of different synthetic strategies for the preparation of boron-rich neutral systems Abstract: Fluorescent star-shaped mol- ecules and dendrimers with a 1,3,5-tri- phenylbenzene moiety as the core and 3 or 9 carborane derivatives at the pe- riphery, have been prepared in very good yields by following different ap- proaches. One procedure relies on the nucleophilic substitution of Br groups in 1,3,5-tris(4-(3-bromopropoxy)phe- nyl)benzene with the monolithium salts of methyl and phenyl-o-carborane. The second method is the hydrosilylation reactions on the peripheral allyl ether functions of 1,3,5-tris(4-allyloxy-phe- nyl)benzene and 1,3,5-tris(4-(3,4,5-tris- ACHTUNGTRENNUNGallyloxyACHTUNGTRENNUNGbenzyloxy)ACHTUNGTRENNUNGphenyl)benzene with suitable carboranyl-silanes to produce different generations of dendrimers decorated with carboranyl fragments. This approach is very versatile and allows one to introduce long spacers between the fluorescent cores and the boron clusters, as well as to obtain a high loading of boron clusters. The re- moval of one boron atom from each cluster leads to high-boron-content water-soluble macromolecules. Ther- mogravimetric analyses show a higher thermal stability for the three-function- alized compounds than for those con- taining 9 clusters. All compounds ex- hibit photoluminescent properties at room temperature under ultraviolet ir- radiation with high quantum yields; these depend on the nature of the clus- ter and the substituent on the C cluster . Cyclic voltammetry indicates that there is no electronic communication be- tween the core and the peripheral car- boranyl fragments. Due to the high boron content of these molecules, we currently focus our research on their biocompatibility, biodistribution in cells cultures, and potential applications for boron neutron capture therapy (BNCT). Keywords: boron · carboranes · cluster compounds · dendrimers · macromolecules · photolumines- cence [a] Dr. A. GonzƁlez-Campo, A. Ferrer-Ugalde, + Prof. C. ViÇas, Prof. F. Teixidor, Dr.R. NfflÇez Institut de Ciŕncia de Materials de Barcelona (ICMAB-CSIC) Campus U.A.B., 08193 Bellaterra, Barcelona (Spain) Fax: (+ 34) 935805729 E-mail: [email protected][b] Prof. R. SillanpȨȨ Department of Chemistry University of JyvȨskylȨ, 40014 JyvȨskylȨ (Finland) [c] J. Rodrȷguez-Romero, Dr. R. Santillan Departamento de Quȷmica Centro de InvestigaciɃn y de Estudios Avanzados del IPN, 07000 Apdo. Postal 14-740, MȖxico D.F. (MȖxico) [d] Dr. N. FarfƁn Facultad de Quȷmica Departamento de Quȷmica OrgƁnica Universidad Nacional AutɃnoma de MȖxico ACHTUNGTRENNUNG(UNAM), MȖxico D.F. 04510 (MȖxico) [ + ] Enrolled in the UAB PhD program. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201203771. Chem. Eur. J. 2013, 19, 6299 – 6312 # 2013 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 6299 FULL PAPER
Transcript
DOI: 10.1002/chem.201203771
A Versatile Methodology for the Controlled Synthesis of PhotoluminescentHigh-Boron-Content Dendrimers
Ar�ntzazu Gonz�lez-Campo,[a] Albert Ferrer-Ugalde,[a] Clara ViÇas,[a]
Francesc Teixidor,[a] Reijo Sillanp��,[b] Jesffls Rodr�guez-Romero,[c] Rosa Santillan,[c]
Norberto Farf�n,[d] and Rosario NfflÇez*[a]
Introduction
The search for luminescent materials is continuously ex-panding due to their applications as light-emitting diodes,semiconductor lasers, probes, or fluorescence sensors,among others.[1]In boron chemistry, there are few reportsconcerning the photophysical properties of boranes and car-boranes. Nevertheless, during the last few years, the interestin the development of new photoluminescent materials thatincorporate polyhedral boron clusters has increased, asshown by the incorporation of these compounds, which pos-sess special electronic properties and geometry, into fluores-cent systems.[2] For this reason, different fluorescent car ACHTUNGTRENNUNGbo-ACHTUNGTRENNUNGrane-appended star-shaped molecules, in which the car ACHTUNGTRENNUNGbo-ACHTUNGTRENNUNGrane is linked to a fluorescent p-conjugated core,[3] to p-con-jugated organic systems,[4] or polymers,[5,6] have been recent-ly described, and the influence of the cluster on theemission properties studied.
On the other hand, the interest in the use of differenttypes of platforms for the incorporation of boron-based clus-ters to obtain high-boron-content molecules for boron neu-tron capture therapy (BNCT), or for drug delivery, has in-creased enormously.[3,7] During the last decade, our grouphas contributed to the development of different syntheticstrategies for the preparation of boron-rich neutral systems
Abstract: Fluorescent star-shaped mol-ecules and dendrimers with a 1,3,5-tri-phenylbenzene moiety as the core and3 or 9 carborane derivatives at the pe-riphery, have been prepared in verygood yields by following different ap-proaches. One procedure relies on thenucleophilic substitution of Br groupsin 1,3,5-tris(4-(3-bromopropoxy)phe-nyl)benzene with the monolithium saltsof methyl and phenyl-o-carborane. Thesecond method is the hydrosilylationreactions on the peripheral allyl etherfunctions of 1,3,5-tris(4-allyloxy-phe-nyl)benzene and 1,3,5-tris(4-(3,4,5-tris-ACHTUNGTRENNUNGallyloxy ACHTUNGTRENNUNGbenzyloxy) ACHTUNGTRENNUNGphenyl)benzene withsuitable carboranyl-silanes to producedifferent generations of dendrimers
decorated with carboranyl fragments.This approach is very versatile andallows one to introduce long spacersbetween the fluorescent cores and theboron clusters, as well as to obtain ahigh loading of boron clusters. The re-moval of one boron atom from eachcluster leads to high-boron-contentwater-soluble macromolecules. Ther-mogravimetric analyses show a higherthermal stability for the three-function-alized compounds than for those con-
taining 9 clusters. All compounds ex-hibit photoluminescent properties atroom temperature under ultraviolet ir-radiation with high quantum yields;these depend on the nature of the clus-ter and the substituent on the Ccluster.Cyclic voltammetry indicates that thereis no electronic communication be-tween the core and the peripheral car-boranyl fragments. Due to the highboron content of these molecules, wecurrently focus our research on theirbiocompatibility, biodistribution in cellscultures, and potential applications forboron neutron capture therapy(BNCT).
[a] Dr. A. Gonz�lez-Campo, A. Ferrer-Ugalde,+ Prof. C. ViÇas,Prof. F. Teixidor, Dr. R. NfflÇezInstitut de Ci�ncia de Materialsde Barcelona (ICMAB-CSIC)Campus U.A.B., 08193Bellaterra, Barcelona (Spain)Fax: (+ 34) 935805729E-mail : [email protected]
[b] Prof. R. Sillanp��Department of ChemistryUniversity of Jyv�skyl�, 40014Jyv�skyl� (Finland)
[c] J. Rodr�guez-Romero, Dr. R. SantillanDepartamento de Qu�micaCentro de Investigaci�n y deEstudios Avanzados del IPN, 07000Apdo. Postal 14-740, M�xico D.F. (M�xico)
[d] Dr. N. Farf�nFacultad de Qu�micaDepartamento de Qu�mica Org�nicaUniversidad Nacional Aut�noma de M�xicoACHTUNGTRENNUNG(UNAM), M�xico D.F. 04510 (M�xico)
[+] Enrolled in the UAB PhD program.
Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201203771.
that incorporate closo-carboranes, and water-soluble anionicmacromolecules functionalized with nido-carborane and co-baltabisdicarbollide fragments, which provide potential bio-medical applications.[8–10] Apart from the high boron con-tent, another interesting property that we have consideredimportant for their biological application is their photophys-ical properties. For this reason, we have prepared Fr�chet-type anionic carboranyl-functionalized aryl-ether derivativesthat exhibit blue emission under ultraviolet irradiation indifferent solvents at room temperature,[11] as well as otherdendrimeric systems that consist of a 1,3,5-triphenylbenzeneunit as fluorescent core,[12,13] poly(aryl-ether) fragments asconnecting groups, and three, six, and twelve terminal cobal-tabisdicarbollide anions.[14] Nevertheless, in the latter, de-spite the fluorescence of the starting dendrimers, after func-tionalization with the metallacarborane, a quenching of thefluorescence was observed. Based on these results, and con-sidering our interest in the preparation of fluorescent boron-rich systems, we wondered if the carborane clusters, unlikethe cobaltabisdicarbollide, would produce an improvementof the photoluminescent properties. For that reason, hereinwe report on the synthesis and characterization of a set offluorescent high-boron-content star-shaped molecules anddendrimers by using a versatile methodology to control andincrease the number of boron atoms. Control of the spacerbetween the core and the peripheral cages is also sought.The crystal structure of one of the carboranyl-containingstar-shaped molecules has been analyzed by X-ray diffrac-tion and is reported herein. The thermal and electrochemi-cal behavior of the compounds has been studied. A com ACHTUNGTRENNUNGpar-ACHTUNGTRENNUNGa ACHTUNGTRENNUNGtive revision of the photoluminescent properties for allcompounds has been carried out, and the influence of thesubstituent attached to the second Ccluster (Me, Ph), the typeof spacer, and the number and nature of the clusters havebeen investigated.
Results and Discussion
Synthesis of compounds 1–27: A set of star-shaped mole-cules and dendrimers with 1,3,5-triarylbenzene as core andincorporating different terminal groups (see Figure 1, 1–3),has been used as platform to prepare fluorescent high-boron-content macromolecules by functionalization withcarboranyl fragments. The synthesis of 1,3,5-tris(4-(3-bromo-propoxy)phenyl)benzene (1) has been described before,[15]
however we used a modified procedure by mixing the corre-sponding 4-(3-bromopropoxy)acetophenone and tetra ACHTUNGTRENNUNGchlo-ACHTUNGTRENNUNGro ACHTUNGTRENNUNGsilane at room temperature. On the other hand, fluores-cent compounds 2 and 3 with terminal allyl ether groupshave already been reported by our group, as well as their re-action with cobaltabisdicarbollide units.[14] In that previouswork, when the periphery of 2 was functionalized with me-ACHTUNGTRENNUNGtallacarboranes, a quenching of the fluorescence occurredand we were not able to explain this phenomenon. Besides,a complete periphery functionalization with cobaltabisdicar-
bollide clusters was not possible for 3, which was attributedto steric hindrance.
Thus, because we are interested in the synthesis of fluo-rescent high-boron-content systems, we explore here thefunctionalization of these fluorescent starting compounds byusing carborane clusters instead of metallacarboranes andstudy their effect on the observed photoluminescent proper-ties.[16]
The different terminal groups on the periphery of thestarting compounds 1–3 allow their functionalization by nu-cleophilic substitution or by catalyzed hydrosilylation reac-tions, respectively. First, the carborane-containing star-shaped molecules 4 and 5 were synthesized by nucleophilicsubstitution of the bromine groups in 1,3,5-tris(4-(3-bromo-propoxy)phenyl)benzene (1), performing the reaction of 1with 3 equivalents of the monolithium salts of 1-Me-1,2-C2B10H11 and 1-Ph-1,2-C2B10H11,
[17] respectively, in THFheated at reflux overnight (Scheme 1).
Figure 1. Starting star-shaped molecules and dendrimers with differentterminal groups.
Scheme 1. Synthesis of 4 and 5 by nucleophilic substitution.
Compounds 2 and 3 were functionalized with differentcarboranyl derivatives by catalyzed hydrosilylation reac-tions. This approach permits a higher versatility and allowsus to introduce longer spacers between the fluorescent coreand the boron clusters, as well as to further increase thenumber of carborane clusters. For that purpose, two differ-ent sets of carboranyl-containing hydrosilylating agents, 6and 7[9a] and 8 and 9, have been used (Figure 2).
Compounds 8 and 9 havebeen prepared in two stepsfrom two recently developedfluorescent styrene-containingcarborane clusters.[18] In thefirst step, a regiospecific hydro-silylation of styrene-containingcarborane clusters with anexcess of HSiMe2Cl at RT, inthe presence of Karstedt cata-lyst, led to the formation of thecorresponding chlorosilane de-rivatives. The hydrosilylation isthe addition of one Si�H func-tion to a double- or triple bondcatalyzed by a transition metal,usually a Pt catalyst, [Pt ACHTUNGTRENNUNG(dvds)](dvds= 1,3-divinyltetramethyl-disiloxane). Karstedts catalystis one of the most regioselectivecatalysts favoring the anti-Mar-kovnikov addition to obtainmainly the b-adduct. However,according to the 1H NMR spec-tra for 8 and 9, a mixture of iso-mers resulted from a- (20 %)and b-additions (80 %); thisresult was different to that of compounds 6 and 7, whichwere obtained as 100 % b-adducts. To control the catalytichydrosilylation and produce only the desired isomer, a new[Pt ACHTUNGTRENNUNG(dvds)]/P ACHTUNGTRENNUNG(tBu)3 complex was used.[19] Thus, the reactionof the styrene-containing carborane clusters with an excessof HSiMe2Cl at RT in the presence of the [Pt ACHTUNGTRENNUNG(dvds)]/P ACHTUNGTRENNUNG(tBu)3
catalyst led to the formation of the corresponding chlorosi-lane derivatives as 100 % b-adducts (Scheme 2). Followingevaporation of the solvents, the second step consists of thereduction of the Si�Cl to Si�H by the addition of LiAlH4 inEt2O at RT to obtain compounds 8 and 9 in high yield(Scheme 2).
To functionalize the periphery of the fluorescent star-shaped molecule 2 and dendrimer 3 with carborane clusters,hydrosilylation reactions of the allyl ether with the hydrosil-ACHTUNGTRENNUNGylating agents 6–9 were carried out in the minimum amountof dry THF at room temperature in the presence of Karstedtcatalyst. The reaction of 3 equivalents of 6–9 with 1 equiva-lent of 2 leads to the three-functionalized molecules 10–13(Scheme 3).
Following the same procedure and to increase the amountof boron, 9 equivalents of 6–9 were mixed with 1 equivalentof 3 to produce compounds 14–17 (Scheme 4). Hydrosilyla-tion reactions were monitored by 1H NMR spectroscopy todetermine the completion of the reaction upon the disap-pearance of the allyl proton resonances of the starting largemolecules (Scheme 4).
The preparation of pure compounds is a requirement forthe development of materials, particularly for biological ap-plications. Therefore, due to the presence of the Karstedtcatalyst, a further purification of the functionalized mole-cules by TLC was necessary. Subsequently, all of the com-
Figure 2. Hydrosilylation agents 6–9.
Scheme 2. Synthesis of 8 and 9 by hydrosilylation reaction and reductionwith LiAlH4.
Scheme 3. Synthesis of three-functionalized compounds 10–13.
pounds (10–13 and 14–17) were obtained with yields in therange 60–85 %.
Preparation of the water-soluble compounds 18–27: Anioniccarborane clusters have also shown potential for differentapplications due to their solubility in water depending onthe counterion used. Indeed, K+ and Na+ have been broad-ly used as counterions for anionic carborane clusters be-cause of the biocompatibility and solubility of these clustersalts in water. To obtain the corresponding carboranyl-con-taining anionic dendrimers, the well-known partial degrada-tion method for closo-carboranes through nucleophilicattack was performed, by using an excess of NaOH in de-oxygenated EtOH. The corresponding polyanionic macro-molecules functionalized with nido-carboranes 18–27 wereisolated as Na+ salts after the work-up. Polyanionic den-
drimers 18–27 are water-soluble compounds and are there-fore good candidates to explore their application as boron-delivery platforms (Scheme 5 and the Supporting Informa-tion).
Characterization of compounds 1–27: The structures of allthe compounds were established by IR, 1H, 13C{1H}, 11B,11B{1H}, 29Si{1H} NMR spectroscopies, elemental analysis,ESI-MS, UV/Vis and fluorescence spectroscopy (and for 4confirmed by X-ray diffraction analysis). The IR spectra ofthe closo-derivatives, 8–17, present the typical v ACHTUNGTRENNUNG(B�H)strong bands for closo-clusters between 2554 and 2584 cm�1.For 8 and 9, a band around 2112 cm�1 corresponding tov ACHTUNGTRENNUNG(Si�H) from the silane function is also observed, which dis-appeared in compounds 10–17 after the hydrosilylation reac-tion, indicating total conversion of the alkene. In the
Scheme 4. Synthesis of nine-functionalized compounds 14–17.
1H NMR spectra of 8 and 9, the resonances due to the vinylprotons have disappeared and new resonances are observedin the aliphatic region that indicate b-addition of the Si�Hto the vinyl.[9a] In addition, one septuplet at aboutd= 3.90 ppm attributed to the Si�H and a doublet close tod= 0.0 ppm from Si�CH3 confirm the formation of the com-pounds. The starting compounds 2 and 3 show resonances inthe range from d=6.20 to 5.20 ppm attributed to the allylprotons.[14] These resonances disappear after the hydrosilyla-tion reaction, indicating the anti-Markovnikov addition ofthe -m-SiH function of 6–9 to the double bonds, and subse-quently the complete peripheral functionalization. In thelatter compounds, the presence of new �CH2� proton reso-nances corroborates their formation (Figure 3). The1H NMR spectra for dendrimers bearing closo-clusters alsoexhibit resonances at low frequencies, in the d=�0.11–0.08 ppm range for Cc-SiCH3 protons. The 13C{1H} NMRspectra show different resonances in the aromatic region,from d=160.5 to 114.0 ppm for all compounds. After func-tionalization with carboranes, the dendrimers show resonan-ces in the region d=84.0 to 74.6 ppm attributed to the
Ccluster. The resonances for the Si�CH3 units bonded to theCc appear around d=�3.5 ppm, whereas the�CH2� carbonsare displayed in the range d= 40.0 to 10.0 ppm.
The 11B{1H} NMR resonances for molecules decoratedwith closo-clusters (4, 5, and 10–17) appear in the region
Scheme 5. a) Deboronation reaction for compounds 10 and 11 and b) deboronation reaction for compounds 16 and 17.
characteristic for closo-compounds, from d=�3.0 to�11.0 ppm. In general, all compounds present broad over-lapped bands with the patterns 2:8, 1:1:8, or 1:1:4:4. Anioniccompounds (18–27) that contain nido-clusters show broadbands in the region from d=�7.0 to �37.0 ppm. All com-pounds show only one peak in the 29Si{1H} NMR spectra,due to the Si nuclei present in the molecules. Compounds 8and 9 containing a �SiH function show a peak atd=�12.80 ppm, very similar to that found for 6–7 atd=�14.10 ppm.[9a] After derivatization, the 29Si{1H} resonan-ces appear around d=+ 3.0 ppm. Different mass spectrome-try techniques (ESI and MALDI-TOF) have been used forthe characterization of compounds. The formula of thesmaller dendrimers was well established by using ESI-MSshowing the molecular ion peak; nevertheless, for the largestdendrimers neither the ESI nor the MALDI-TOF massspectra were useful, because an important fragmentationwas observed. A similar fragmentation had been previouslyobserved for other boron-containing large molecules.[10]
Crystal structure of 4 : Compound 4 crystallizes with onemolecule of hexane. The bonding parameters of 4 do notshow any unusual features. The geometry of 4 is shown inFigure 4. The three phenyl groups (C1a-C6a, C1b-C6b and
C1c-C6c) attached to the central phenyl group (C1-C6) areslightly tilted from the plane of that group (by 30.5(1),50.6(1) and 34.0(1) 8), respectively. The packing of 4 is con-trolled by weak van der Waals interactions. The packingview, presented in Figure 5, shows a layer-type structurewith hexane molecules in the closed holes.
Thermal properties : The utilization of carborane clusters forthe preparation of thermally stable materials has been ex-plored to improve their final thermal properties. Previously,we had reported an exhaustive study and thermal evolutionof carborane-containing sol–gel hybrid materials, showingan enhancement of the thermal stability of xerogels.[20]
Other silicon-derived polymers, coordination polymers, con-ducting organic polymers, or high carboranyl-containing
macromolecules have been used as oxidative protectioncoatings or ceramic polymeric materials that exhibit out-standing thermal and thermo-oxidative properties.[3] Tostudy the thermal stability of the carboranyl-containing mac-romolecules, different parameters have been considered: thepresence of the Si�C bond, the number of clusters, and thepercentage boron content (Figure 6). Compounds 4, 10, and12, which contain three cages, are shown to be thermallystable up to 350 8C, however 14 and 15 with nine clustersstart to lose mass at lower temperatures.
By examining the thermogravimetric analysis (TGA)curves of the tri-functionalized 4 and 10 under N2, it can beconcluded that the presence of the Si�C bond in 10 doesnot affect the thermal stability of the dendrimers. Both start-ed to lose weight around the same temperature, 395 and385 8C, respectively. Nevertheless, some differences between10 and 12 are observed; compound 12 is less stable and itsthermal decomposition occurs in two steps: the first one, be-tween 200 and 390 8C, results in a mass loss of a 10 % thatcan be due to the decomposition of the peripheral aromatic
Figure 5. Packing of 4·hexane showing the location of hexane moleculesin the holes.
Figure 6. Thermogravimetric analysis (TGA) curves of different dendri-ACHTUNGTRENNUNGmers 4, 10, 12, 14, and 15 under nitrogen.
Figure 4. The asymmetric unit of 4·hexane. The hydrogen atoms and thehexane molecule have been omitted for clarity reasons. Thermal ellip-soids have been presented at the 20% probability level.
ring bonded to the carborane cages; the second step sets upat 390 8C, with a higher decomposition, to give a residue of17.1 % In addition, for the star-shaped macromolecules with3 clusters (4, 10, and 12) the results observed are consistentwith their boron content (Table 1). Compound 4, which is
the most stable of the three compounds and shows the high-est final residual mass, also has the highest percentage ofboron content (34.2 %), whereas 10 and 12 have 25.9 % and21.9 % boron by weight (wt), respectively. When the boroncontent (wt %) of the macromolecules decreases, the ther-mal stability decreases as well. Both nine-functionalizeddendrimers, compound 14 with nine Me-o-carborane and 15with nine Ph-o-carborane, are less thermally stable and themass loss occurs at about 300 8C with a relative higher finalresidual mass. For 14, with 28.2 % boron content, the finalresidual mass is 32.5 %, whereas for 15 a 36.7 % is recoveredat 600 8C. This phenomenon can be attributed to an increaseof the organic fragments due to the new �OCH2� groupsand the number of alkyl chains. The final residual mass ofboth compounds is in agreement with the wt % boron con-tent, although for 15 a higher residue is obtained. This resultindicates that when Ph-o-carborane is used other effectsmay influence the thermal behavior of the dendrimer be-cause even with lower wt % boron the final mass residue isstill higher than those with a higher boron content.
Dendrimers 12, 14, and 15 start to lose weight earlier that4 and 10, this phenomenon is similar to that observed inother macromolecules and branched polymers that containbranching units, apparently decompositions starts in thesebranches. However, in all our cases an important percentageof the residue is maintained due to the presence of the clus-ters in the molecule.
Photophysical properties : The UV/Vis absorption spectra ofcompounds 1–5 and 10–17 were performed in acetonitrileand THF, whereas compounds 18–27 were measured inH2O. Tables 2 and 3 list the spectroscopic and photophysicaldata obtained for these compounds. The starting molecules,carboranyl-functionalized star-shaped molecules and den-drimers, 1–5 and 10–27, display a bathochromic shift and ex-hibit bands of absorption maxima in the region 266–275 nm,which correspond to the p–p* transitions in the aromaticcore, which are 10–20 nm redshifted with respect to themaximum at 254 nm reported for 1,3,5-triphenylsubstitutedbenzene compounds.[13d] In general, neither changes on thesubstituent at the Ccluster nor modifications in the electronic
properties of the cluster (closo and nido) cause substantialalterations of the absorption wavelengths. This indicates thatthe absorption properties are dominated by the core as thechromophore, and the introduction of carborane clustersdoes not influence them.
Figure 7 shows absorption and emission bands of allyl-ter-minated 2 and carboranyl functionalized star-shaped mole-cules 10–13 in acetonitrile, whereas Figure 8 exhibits absorp-
tion and emission spectra of dendrimers that contain ninefunctional groups 14–17 in THF. As is usually the case, theexcitation spectra of 1–5 and 10–27 resemble their absorp-tion spectra. All compounds exhibited emission bands in theblue-violet region, with maximum emission intensities (lem)around 366 nm due to the local emission of the chromo-phore. These maxima lem are not influenced by the solvent(Table 2), therefore no solvatochromic effect is observed.
These results are just the opposite to those previously re-ported by us, in which similar dendrimeric systems were pre-
Table 1. Percentage boron content, mass loss temperatures and final resi-dues [%] from TGA.
pared, but in that case the cores were decorated with anion-ic cobaltabisdicarbollide fragments resulting in the quench-ing of the fluorescence.[14] To measure the fluorescencequantum yields (FF), compounds 1 and 4 and 5 were excitedat 266 nm, whereas the rest of the compounds were excitedat 269 nm. The FF values were calculated by using quininesulphate in an aqueous solution of H2SO4 (0.5 m) as a stand-ard (see Table 2).
To study the influence of the carboranyl fragments on thephotoluminescent properties, different compounds havebeen prepared by changing: 1) the substituent at the Ccluster
(Me or Ph); 2) the distance and the type of spacer betweenthe cluster and the chromophore, as well as the generationof the dendrimer; 3) the nature of the cluster, either hydro-phobic closo-clusters or hydrophilic nido-clusters; and 4) thenumber of carboranyl groups.
The substituent at the Ccluster : As a general trend, and inde-pendently of the way in which the carboranyl fragment isbonded to the starting molecule, the incorporation ofmethyl-o-carborane produces an enhancement of the emis-sion intensity with respect to the starting compound, withquantum yields ranging from 47 to 59 % (Table 2). Con-versely, dendrimers bearing phenyl-o-carborane clusters ex-hibited lower fluorescence intensity, with relative quantumyields of 13–26 % in acetonitrile, and 11–32 % in THF(Table 2). These differences can be clearly observed in Fig-ures 7 and 8, in which the emission spectra of compounds10–13 and 14–17 are represented. Compounds 10, 12, 14,and 16, bearing methyl-o-carborane, show higher relativequantum yields than compounds 11, 13, 15, and 17, whichcontain phenyl-o-carborane. These results could be ex-plained by considering our previous work on the photolumi-nescence properties of carboranyl-containing styrene deriva-tives, in which it was reported that the introduction of aphenyl group into the electron-withdrawing carboranyl frag-ment results in an excellent electron-acceptor unit that uponbeing covalently bonded to an electron-donor produces aquenching of the fluorescence.[18] This quenching is the
result of a charge-transfer process from the styrene to thephenyl-carborane unit. In the current compounds, only par-tial quenching of the fluorescence was observed, howeverthe decrease of the emission intensities for the phenyl-o-car-borane with respect to the methyl-o-carborane derivativescould be due to a process in which the phenyl group wouldbe involved.
Distance/type of spacer and generation of the dendrimer : Forcompounds bearing methyl-o-carborane, the fluorescence in-tensities are very similar and the influence of the distanceand type of spacer between the cluster and the chromophoreis practically unnoticed in both THF and acetonitrile. Never-theless, for those molecules containing phenyl-o-carboraneat the periphery, an important effect of these two factors isobserved. If the compounds with 3 clusters are considered(Figure 9), compound 11 is the one with a higher fluores-
cence intensity (32 % of FF), whereas 5 and 13 show lowerrelative quantum yields (23 and 24 %, respectively). This dif-ference could be due to electron-transfer processes causedby the existence of the phenyl-o-carborane, as has been pre-viously discussed,[18] which also depends on the type ofspacer.[21] In the case of compounds with 9 clusters, despitethe longer spacer that increases the distance between thecluster and the chromophore, the relative quantum yields of15 and 17 (24 and 11 %) are reduced in relation to their ana-logues with 3 clusters (Figure 9). This effect could be ex-plained by the fact that after growing the molecule, six addi-tional phenyl rings are introduced to the chromophore,which could facilitate other interactions, such as p···p inter-actions, weakly B�H···H�Caryl interactions, among others.This interpretation can be supported by the insolubility of17 in acetonitrile, this being the reason why this data is notavailable.
The nature of the cluster : Concerning the nature of the clus-ter, the largest fluorescence quantum yields are always
Figure 8. Emission and normalized absorption data for 3 and the nine-functionalized 14–17 in THF.
Figure 9. Emission spectra in THF for different compounds bearingphenyl-o-carborane clusters.
found for closo-compounds, whereas aqueous solutions ofnido-derivatives 20–26 show comparatively much lowerquantum yields, between 9 and 23 % (Table 3). These resultsare in agreement with previously reported data,[3,18] confirm-ing that the presence of the hydrophilic nido-carborane clus-ters produces a decrease of the emission intensities(Figure 10).
Nevertheless, it is important to notice that the relativequantum yields of 20–26 in water are reasonably good ifthey are compared with the previously reported star-shapedmolecules, which were remarkably lower.[3a] We have attrib-uted this difference to a longer distance between the nido-carborane clusters and the chromophore, since the increaseof the number of charged clusters close to the chromophorediminishes the fluorescence quantum yields.[3a]
The number of carboranyl groups : The introduction of alarger number of homologous neutral closo-carborane clus-ters into the molecule does not seem to affect the fluores-cence behavior of the dendrimers, because the fluorescencequantum yields of 10 and 14, or 12 and 16 are very similar.The largest difference was found for 17, with a lower FF
with respect to its homologous 13. On the other hand, the
increase in the number of hydrophilic nido-carboraneswithin the molecule provides a better water solubility forthe chromophore, but does not affect in general the fluores-cence intensity (Table 3). This is a good result because it isknown that most water-soluble chromophores show low flu-orescence due to the photoinduced electron-transfer inpolar solvents.[22]
Electrochemical studies : Cyclic voltammetry (CV) has beenutilized to understand the degree of electronic communica-tion between the triarylbenzene core and the peripheral car-boranes separated with 4, 7, or 11 non-conjugated atomspacers. The ratio of carboranes to the triarylbenzene coreranges from 3 in compounds 10–13, to 9 for compounds 14–17. The CVs of compounds 1 and 2, the first terminatedwith Br and the second with allyl groups, show no redox ac-tivity in THF (See the Supporting Information). Conversely,the methyl- and phenyl-o-carborane are redox active andhave been taken as references. Preliminary CV studies wereperformed from +3 to �3 V versus SCE. The phenyl-o-car-borane has displayed a reduction potential around �2.2 Vand the methyl near �2.9 V. These values are close to thosedescribed in the literature at �1.95 and 2.44 versus SCEfrom polarographic data;[23] probably this corresponds onlyto 1e� reduction of the cage, the difference may result fromusing THF instead of acetonitrile or DMF. The CVs of the1,3,5-triarylbenzene dendrimers peripherically decoratedwith methyl-o-carboranes or phenyl-o-carboranes are verysimilar to those obtained for the isolated carboranes inTHF. These results indicate that the core does not influencethe electroactivity of the peripheral carboranes, so that the1,3,5-triarylbenzene core and the carboranes are not elec-tronically communicated. This implies that their individualentities in the dendrimers or star-shaped molecules arelargely preserved. Therefore, these results corroborate thatthe absorption and emission data grossly remain unaltered.
Conclusion
A new family of star-shaped molecules and dendrimers thathave the 1,3,5-triphenylbenzene unit as fluorescent core, andthree- or nine terminal carboranyl fragments, have been pre-pared by using various approaches. The methodology used ishighly versatile and allows one to introduce different spacersbetween the nuclei and the boron clusters. It also facilitatesto further increase the number of carborane clusters, as wellas to modify their nature either neutral or anionic. Thermalstability studies of the compounds indicate that star-shapedmolecules with three cages are more stable than dendrimerscontaining nine terminal clusters. A high percentage of resi-due is obtained after thermal experiments due to the pres-ence of the carboranes. This quantity of residue fits wellwith the wt % of boron in the molecules. All compounds ex-hibit photoluminescence at room temperature in the blue-violet region and relative high quantum yields, even for theanionic species; these depend on the nature of the cluster
Table 3. Spectroscopic data of star-shaped molecules and polyanionicdendrimers.
and the second substituent on the Ccluster. None of the com-pounds reported here show total quenching of the fluores-cence; these results conflict with other earlier reports for an-alogue dendrimers functionalized with metallacarboranes. Inagreement with the photoluminescent results, the cyclic vol-tammetry studies have shown that the two electroactive cen-ters, the 1,3,5-triarylbenzene and the carborane, are notelectronically communicated; this corroborates that the ab-sorption and emission wavelengths are practically unaffect-ed.
Experimental Section
Synthesis of 1,3,5-tris(4-(3-bromopropoxy)phenyl)benzene (1): 4-(3-bro-mopropoxy)acetophenone (1.00 g, 3.68 mmol) and tetrachlorosilane(5.64 g, 33.19 mmol) were mixed in absolute ethanol at room temperaturefor 48 h. The solid obtained was filtered and washed with methanol togive 1 as a white powder. Yield: 0.74 g, 80.0 %. M.p. 115–117 8C;1H NMR (CDCl3): d=7.65 (s, 3H; C-Haryl), 7.62 (d, 3J ACHTUNGTRENNUNG(H,H) =8.7 Hz,6H; C-Haryl), 7.03 (d, 3J ACHTUNGTRENNUNG(H,H) =8.7 Hz, 6 H; C-Haryl), 4.16 (t, 3J ACHTUNGTRENNUNG(H,H) =
found: 1135.7; elemental analysis calcd for C57H78B30O3: C 60.29, H 6.92;found: C 60.25, H 6.86.
Synthesis of 8 : In a Schlenk flask, 1-(CH2C6H4-4’-(CH2=CH2))-2-CH3-1,2-closo-C2B10H10 (200 mg, 0.70 mmol), HSiMe2Cl (1 mL, 9 mmol) and[Pt ACHTUNGTRENNUNG(dvds)]/PACHTUNGTRENNUNG(tBu)3 catalyst system (10 mL, 0.02 mmol) were mixed andstirred for 6 h at room temperature. Volatiles and the excess of(CH3)2HSiCl were evaporated in the vacuum lines to obtain a yellowishoil that was dissolved in diethyl ether (10 mL) and LiAlH4 (27 mg,0.70 mmol) were added. The mixture was stirred overnight at room tem-perature and filtered off through Celite twice. The solvent was removedin vacuo to give 8 as transparent oil. Yield: 218 mg, 89%. 1H NMR(CDCl3): d=7.21 (d, 3J ACHTUNGTRENNUNG(H,H) = 9 Hz, 2H; C-Haryl), 7.12 (d, 3J ACHTUNGTRENNUNG(H,H) =
st), 2582 (s, B-H st), 2112 cm�1 (s, Si-H st); MS (MALDI-TOF): m/zcalcd for C19H32B10Si: 396.40; found: 397.45 [M+H]+; elemental analysiscalcd for C19H32B10Si: C 57.60, H 8.10; found: C 56.94, H 7.94.
Synthesis of 10 : In a Schlenk flask, 1-CH3-2-[CH2CH2CH2 ACHTUNGTRENNUNG(CH3)2SiH]-1,2-C2B10H10 (6) (0.37 g, 1.43 mmol), 1,3,5-tris(4-allyloxyphenyl)benzene (2)(0.23, 0.48 mmol), Karstedt catalyst (10 mL) and dry THF (1 mL) weremixed and stirred at room temperature for 60 h. The volatiles were re-moved under pressure to obtain a brown oil, which was purified by prep-arative TLC (ethyl acetate/hexane 1:2) and washed with hexane to obtaincompound 10 as a white solid. Yield: 0.52 g, 60.6 %. 1H NMR (CDCl3):d=7.62 (m, 9H; C-Haryl), 7.01 (d, 3J ACHTUNGTRENNUNG(H,H) =9.0 Hz, 6 H; C-Haryl), 4.01 (t,3J ACHTUNGTRENNUNG(H,H) =6.0 Hz, 6H; O-CH2), 2.19 (t, 3J ACHTUNGTRENNUNG(H,H) =9.0 Hz, 6H; Cc-CH2-),2.01 (s, 9 H; Cc-CH3), 1.83 (m, 6 H; CH2-CH2-CH2), 1.57 (m, 6H; CH2-CH2-CH2), 0.69 (t, 3J ACHTUNGTRENNUNG(H,H) =9.0 Hz, 6H; Si-CH2), 0.58 (t, 3J ACHTUNGTRENNUNG(H,H) =
130 Hz, 24B); 13C{1H} NMR (CDCl3): d=158.8–114.9 (Caryl), 78.0 (Cc-CH2), 74.9 (Cc-CH3), 70.7 (O-CH2) 40.3 (-CH2-), 29.7 (CH2CH2Si), 23.8(CH2CH2CH2), 23.6 (Cc-CH3), 16.8 (SiCH2), 10.8 (SiCH2) �3.3 ppm (Si-CH3); 29Si{1H} NMR (CDCl3): d=2.94 ppm; IR (KBr): n =2931 (m, C-Hst), 2574 (s, B-H st), 1608 (m, arC-C st), 1510 (s, arC-C st), 1248 cm�1 (s,C-O-C st); MS (ESI): m/z calcd for C75H120B30O3Si3: 1478; found: 1557[M+2 K]+; elemental analysis calcd for C75H120B30O3Si3: C 60.93, H 8.18;found: C 61.00, H 8.31.
Synthesis of 13 : The procedure was the same as for 12, using 9 (75 mg,0.19 mmol), 1,3,5-tris(4-allyloxyphenyl)benzene (2) (28 mg, 0.06 mmol)and the Karstedt catalyst system (10 mL, 0.02 mmol). After work up, com-
Synthesis of 15 : The procedure was the same as for 10, using 1-C6H5-2-[CH2CH2CH2 ACHTUNGTRENNUNG(CH3)2SiH]-1,2-C2B10H10 (7) (0.34 g, 1.05 mmol), 1,3,5-tris[4-(3,4,5,-tris-(allyloxy)benzyloxy)-phenyl]benzene (3) (0.13 g, 0.11 mmol),Karstedt catalyst (10 mL), and dry THF (1 mL). After work up, com-pound 15 was obtained as a brownish solid. Yield: 0.47 g, 59.1 %.1H NMR (CDCl3): d=7.65–7.40 (m, 54 H; C-Haryl), 7.10 (d, 3J ACHTUNGTRENNUNG(H,H) =
Synthesis of 19 : A solution of 5 (70.7 mg, 0.06 mmol) in of THF (2 mL)was added to a solution of NaOH (75.0 mg, 1.88 mmol) in deoxygenated
ethanol (5 mL). The mixture was heated at reflux for 16 h. After coolingthe mixture, the excess of NaOH was precipitated as sodium carbonateby saturating the solution with a stream of CO2 (g). After filtration, thesolution was evaporated and treated with cold THF. The precipitate wasfiltered off through Celite and evaporation of the solvent led 19 as a yel-lowish oil. Yield: 57.9 mg, 79.4 %. 1H NMR (CD3COCD3): d=7.69–7.16(m, 24 H; C-Haryl), 6.87 (d, 3J ACHTUNGTRENNUNG(H,H) =8.4, 6H; C-Haryl), 3.64 (t, 3J ACHTUNGTRENNUNG(H,H) =
Synthesis of 21: A solution of 11 (90.0 mg, 0.06 mmol) in THF (2 mL)was added to a solution of NaOH (144.0 mg, 3.60 mmol) in deoxygenatedethanol (5 mL). The mixture was heated at reflux for 17 h. After coolingthe mixture, the excess of NaOH was precipitated as sodium carbonateby saturating the solution with a stream of CO2 (g). After filtration, thesolution was evaporated and treated with cold acetone. The precipitatewas filtered off through Celite and evaporation of the solvent led 21 as ayellowish oil. Yield: 60.0 mg, 65.2 %. 1H NMR (CD3COCD3, TMS): d=
Synthesis of 22 : A solution of 12 (50.0 mg, 0.033 mmol) in THF (1 mL)was added to a solution of NaOH (88 mg, 2.20 mmol) in deoxygenatedethanol (3 mL). The mixture was heated at reflux overnight. After cool-ing the mixture, the excess of NaOH was precipitated as sodium carbo-
nate by saturating the solution with a stream of CO2 (g). After filtrationthe solution was evaporated and treated with cold THF. The precipitatewas filtered off through Celite and evaporation of the solvent led 22 asan orange oil. Yield: 36 mg, 71%. 1H NMR (CD3COCD3): d=7.76 (s,3H; C-Haryl), 7.74 (d, 3J ACHTUNGTRENNUNG(H,H) =6 Hz, 6H; C-Haryl), 7.27 (d, 3J ACHTUNGTRENNUNG(H,H) =
158.9 �114.9 (Caryl), 70.7 (O-CH2) 41.8 (-CH2-), 30.1 (CH2CH2Si), 23.9(CH2CH2CH2), 16.8 (CH2CH2Si), 10.6 (CH2CH2CH2) �3.4 ppm (Si-(CH3)2); IR (NaCl): n =2959–2868 (m, C-H st), 2515 cm�1 (s, B-H st); el-emental analysis calcd for C90H126B27O3Si3Na3: C 63.55, H 7.17; found: C62.77, H 7.92.
Synthesis of 24 : A solution of 14 (150.0 mg, 0.04 mmol) in THF (2 mL)was added to a solution of NaOH (156.0 mg, 3.9 mmol) in deoxygenatedethanol (5 mL). The mixture was heated at reflux for 19 h. After coolingthe mixture, the excess of NaOH was precipitated as sodium carbonateby saturating the solution with a stream of CO2 (g). After filtration thesolution was evaporated and treated with cold acetone. The precipitatewas filtered off through Celite and evaporation of the solvent led 24 asan orange oil. Yield: 97.7 mg, 63.2 %. 1H NMR (CD3COCD3, TMS): d=
Synthesis of 25 : A solution of 15 (100.0 mg, 0.03 mmol) in THF (2 mL)was added to a solution of NaOH (89.7 mg, 2.2 mmol) in deoxygenatedethanol (5 mL). The mixture was heated at reflux for 19 h. After coolingthe mixture, the excess of NaOH was precipitated as sodium carbonateby saturating the solution with a stream of CO2 (g). After filtration thesolution was evaporated and treated with cold THF. The precipitate wasfiltered off through Celite and evaporation of the solvent led 25 as anorange oil. Yield: 74.0 mg, 72.0 %. 1H NMR (CD3COCD3, TMS): d=
Synthesis of 27: A solution of 17 (50.0 mg, 0.010 mmol) in THF (1 mL)was added to a solution of NaOH (31 mg, 0.76 mmol) in deoxygenatedethanol (3 mL). The mixture was heated at reflux overnight. After cool-ing the mixture, the excess of NaOH was precipitated as sodium carbo-nate by saturating the solution with a stream of CO2 (g). After filtrationthe solution was evaporated and treated with cold acetone. The precipi-tate was filtered off through Celite and evaporation of the solvent led 27as an orange oil. Yield: 37 mg, 72%. 1H NMR (CD3COCD3): d=7.74–6.81 (m, 102 H; C-Haryl), 5.00 (s, 6H; O-CH2-Caryl), 3.96 (m, 18H; O-CH2), 3.22 (s, 18H; Cc-CH2), 2.60 (m, 18H; SiCH2CH2C6H4), 1.83 (m,18H; CH2CH2CH2Si), 0.88 (m, 18H; SiCH2CH2C6H4), 0.63 (m, 18H;CH2CH2CH2Si), 0.02 ppm (s, 54H; Si ACHTUNGTRENNUNG(CH3)2); 1H ACHTUNGTRENNUNG{11B} NMR
X-ray structure determinations : Crystallographic data for compound4·hexane was collected at 123 K with a Bruker Nonius-Kappa CCD areadetector diffractometer, using graphite-monochromatized MoKa radiation(l=0.71073 �). The structure was solved by direct methods by use of theSHELXS-97 program.[24] The full-matrix, least-squares refinements on F2were performed using SHELXL-97 program.[24] The non-hydrogen atomswere refined with anisotropic thermal displacement parameters. Hydro-gen atoms were treated as riding atoms using the SHELX97 default pa-rameters.
CCDC-906346 (4·hexane) contain the supplementary crystallographicdata for this paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was supported by the Spanish Ministerio de Ciencia e Innova-ci�n (CTQ2010–16237) and the Generalitat de Catalunya 2009/SGR/00279. A.G.C. and A.F.U. thank the Generalitat de Catalunya for a Bea-triu de Pin�s contract and a FI grant, respectively. J. R. R. thanks to Con-acyt for financial support for a research stay at the ICMAB-CSIC.
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Received: October 10, 2012Revised: January 23, 2012
